A semiconductor laser device, such as a Vertical Cavity Surface-Emitting Laser (VCSEL) device, produces monochromatic, coherent light when electrical current passes through it. Essentially, a VCSEL is made up of a layer of light-emitting material sandwiched between two reflective layers.
Heat dissipation is one of the challenges with which VCSEL designers must deal. The amount of heat generated is related to the resistance of the VCSEL, and to the amount of current the VCSEL carries. The resistance is commonly called "series resistance," because the current flows through the layers in series. While it is desirable that the VCSEL be able to maximize the power output of the laser light by carrying as much current as possible, the heating limits the amount of current that can be carried.
Conventional approaches have been used to reduce the heat dissipation efficiency of the VCSEL structure. For instance, Kish, Jr., et al., U.S. Pat. No. 5,724,376, "Transparent Substrate Vertical Cavity Surface Emitting Lasers Fabricated by Semiconductor Wafer Bonding," referred to below in connection with FIG. 1, describes a structure which employs a heat sink in proximity to the active layer.
It is also known that the resistance of the layers making up the reflective structures can be reduced by doping. This allows more current to flow, for a given amount of heating, so the light power output is increased. See, for instance, K. L. Lear et al., "Low Threshold Voltage Vertical Cavity Surface-Emitting Laser," Electronics Letters, Vol. 29, No. 7, (Apr. 1, 1993), pp. 584-6.
Also, in P. Zhou et al., "Low Series Resistance High-Efficiency GaAs/AlGaAs Vertical-Cavity Surface-Emitting Lasers with Continuously Graded Mirrors Grown by MOCVD", IEEE Photonics Technology Letters, Vol. 3, No. 7 (July 1991), a technique is described for reducing series resistance by grading interfaces between reflector layers.
However, there remains a need for additional new VCSEL structures, which further improve current-carrying capacity while limiting resistance, and which provide good manufacturability.